I've used mice that have been vacuum sealed and frozen for longer than 4 months, and I have never experienced any problems.
Having said that - if I had mice that looked damaged from the freezing process, I would not be using them.
Equally, if they had undergone any complete or partial that due to a freezer mal-function/power cut etc. I wouldn't use them.
I do believe that they may lose some nutritional value with time, but I have yet to read the proof of this, it is just my belief,
Skye